Electrochemical Characterization of Nanoporous Films Fabricated

Nov 1, 2007 - Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, Kansas 66506. Langmuir , 2007, 23 (25), pp 12771–12776...
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Langmuir 2007, 23, 12771-12776

12771

Electrochemical Characterization of Nanoporous Films Fabricated from a Polystyrene-Poly(methylmethacrylate) Diblock Copolymer: Monitoring the Removal of the PMMA Domains and Exploring the Functional Groups on the Nanopore Surface Yongxin Li, Helene C. Maire, and Takashi Ito* Department of Chemistry, Kansas State UniVersity, 111 Willard Hall, Manhattan, Kansas 66506 ReceiVed September 5, 2007. In Final Form: September 18, 2007 Cyclic voltammetry (CV) was used to assess fabrication of a nanoporous film from a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) and also to explore the surface functional groups on the resulting nanopores. Polymer films containing vertically aligned cylindrical nanoscale pores (ca. 10 nm in pore radius, 20-30 nm in film thickness) were prepared on gold substrates by removing the cylindrical PMMA domains from PS-b-PMMA films via UV irradiation and subsequent acetic acid treatment. CV measurements provided a simple means for monitoring the extent of the removal of the PMMA domains and for assessing the formation of a recessed nanodisk-array electrode (RNE) structure. The resulting RNEs exhibited a decrease in redox current of anionic Fe(CN)63- with increasing solution pH from 4.6 to 6.3 and a negligible change in CV of uncharged 1,1′-ferrocenedimethanol. The decrease in redox current of Fe(CN)63- at the higher pH was due to electrostatic repulsion between Fe(CN)63- and the electrical double layer formed in the neighborhood of the negatively charged nanopore surface. Indeed, the reduction of effective pore radius measured from CVs of Fe(CN)63- was correlated to the change in the thickness of the electrical double layer. The pH range that showed the decrease in redox current of Fe(CN)63- was consistent with the presence of -COOH groups on the nanopore surface, although they were not detected using Fourier transform infrared spectra of etched PS-b-PMMA films.

Introduction This paper describes electrochemical characterization of nanoporous films prepared from a poly(styrene)-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) immobilized on a gold substrate. Cyclic voltammetry (CV) was used to assess the completeness of chemical etching of the cylindrical PMMA domains from an annealed PS-b-PMMA film. In contrast to atomic force microscopy (AFM) and Fourier transform infrared-external reflection spectroscopy (FTIR-ERS), CV could detect the extent of exposure of the bottom gold surface to give a recessed nanodiskarray electrode (RNE) structure (Scheme 1). Additionally, CV measurements using charged and uncharged redox species at different solution pH and supporting electrolyte concentrations made it possible to recognize surface functional groups on nanopores prepared from PS-b-PMMA. Recently, block copolymers (BCPs) consisting of two or more chemically distinct polymer fragments have been used to fabricate films containing an array of cylindrical nanoscale pores (nanopores) on planar substrates.1,2 If the constituent polymers are immiscible and the volume fraction of the fragments in a BCP is appropriate, the minor components form cylindrical domains via self-assembly.3 The size of the cylindrical domains is determined by the molecular weight of the polymer.4,5 The orientation of the domains in a BCP film immobilized on a planar substrate has often been controlled by controlling film thickness,2,3,6 controlling the affinity of the BCP components to the * To whom correspondence should be addressed. Telephone: 785-5321451. Fax: 785-532-6666. E-mail: [email protected]. (1) Hillmyer, M. A. AdV. Polym. Sci. 2005, 190, 137-181. (2) Li, M.; Coenjarts, C. A.; Ober, C. K. AdV. Polym. Sci. 2005, 190, 183-226. (3) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323-355. (4) Xu, T.; Kim, H.-C.; DeRouchey, J.; Seney, C.; Levesque, C.; Martin, P.; Stafford, C. M.; Russell, T. P. Polymer 2001, 42, 9091-9095. (5) Guarini, K. W.; Black, C. T.; Milkove, K. R.; Sandstrom, R. L. J. Vac. Sci. Technol. B 2001, 19, 2784-2788.

Scheme 1

supporting substrates via their surface modification,2,7-9 addition of homopolymers,10 control of solvent-evaporation conditions,11 and/or electric field application during annealing.12-14 The resulting cylindrical domains in a BCP film are subsequently (6) Wang, H.; Djurisic, A. B.; Xie, M. H.; Chan, W. K.; Kutsay, O. Thin Solid Films 2005, 488, 329-336. (7) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458-1460. (8) Mansky, P.; Russell, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Macromolecules 1997, 30, 6810-6813. (9) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610-6614. (10) Peng, J.; Gao, X.; Wei, Y.; Wang, H.; Li, B.; Han, Y. J. Chem. Phys. 2005, 122, 114706. (11) Xuan, Y.; Peng, J.; Cui, L.; Wang, H.; Li, B.; Han, Y. Macromolecules 2004, 37, 7301-7307. (12) Thurn-Albrecht, T.; DeRouchey, J.; Russell, T. P.; Jaeger, H. M. Macromolecules 2000, 33, 3250-3253. (13) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787-791. (14) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129.

10.1021/la702756s CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007

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removed by chemical means to obtain nanoporous films.1,2,13-16 BCP-based nanoporous films can be prepared in various shapes and thicknesses, in contrast to other types of nanoporous membranes, such as track-etched membranes and nanoporous anodic alumina membranes.17,18 They have been used as masks for lithography,5,19 as templates for metal or silica nanowire synthesis,14,20,21 for electrophoretic deposition of nanoparticles,22 and for filtration of viruses.23 For these applications, the nanopores formed as a result of the removal of the cylindrical domains need to penetrate the films from one face to the other. So far, atomic force microscopy (AFM),3,4,6,10,11,13,23 Fourier transform infrared (FTIR) spectroscopy,24 and electron microscopy3,5,6,13,23 have been used to characterize BCP-based nanoporous films on planar substrates. However, the former two techniques cannot be used to judge whether the pores go through the film to reach the substrate, and the last technique requires relatively long times for sample preparation and measurements. Electrochemical techniques were used previously to characterize nanopores in etched BCP films,25,26 but they have not been used to monitor nanopore formation during the chemical treatment used to remove cylindrical domains. In addition, considering the significance of interactions between molecules and the nanopore surface for templating and filter applications,14,20,21,23 it is important to have detailed knowledge of the functional groups present on the nanopore surface and also to establish methods to tailor them. Hillmyer et al. reported esterification between trifluoroacetic anhydride and residual -OH groups within nanopores prepared from polystyrene-polylactide diblock copolymers,27 amidation of surface -COOH groups for nanoporous films prepared from polylactide-polydimethylacrylamide-polystyrene triblock copolymers,28 and fabrication of nanoporous materials having derivatizable alkene groups from polystyrene-polyisoprene-polylactide triblock copolymers.29 In these polymers, the nanoporous structures were formed as a result of the hydrolysis of cylindrical polylactide domains; thus, the surface functional groups on the resulting nanopores were predictable. In contrast, to the best of our knowledge, there is no previous report identifying the surface functional groups on BCP-based nanopores formed via oxidative chemical processes, which are widely used to prepare nanoporous structures from polystyrene-poly(methyl methacrylate) (PS-b-PMMA),13,14 polystyrene-polybutadiene,15 and polystyrene-poly(ethylene oxide) diblock copolymers.16 Addressing the surface chemistry within (15) Mansky, P.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Yao, N. Appl. Phys. Lett. 1996, 68, 2586-2588. (16) Mao, H.; Hillmyer, M. A. Macromolecules 2005, 38, 4038-4039. (17) Martin, C. R. Science 1994, 266, 1961-1966. (18) Baker, L. A.; Jin, P.; Martin, C. R. Crit. ReV. Solid State Mater. Sci. 2005, 30, 183-205. (19) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401-1404. (20) Kim, H.-C.; Jia, X.; Stafford, C. M.; Kim, D. H.; McCarthy, T. J.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2001, 13, 795-797. (21) Crossland, E. J. W.; Ludwigs, S.; Hillmyer, M. A.; Steiner, U. Soft Matter 2007, 3, 94-98. (22) Zhang, Q.; Xu, T.; Butterfield, D.; Misner, M. J.; Ryu, D. Y.; Emrick, T.; Russell, T. P. Nano Lett. 2005, 5, 357-361. (23) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. AdV. Mater. 2006, 18, 709-712. (24) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Sibener, S. J. AdV. Mater. 2005, 17, 2446-2450. (25) Jeoung, E.; Galow, T. H.; Schotter, J.; Bal, M.; Ursache, A.; Tuominen, M. T.; Stafford, C. M.; Russell, T. P.; Rotello, V. M. Langmuir 2001, 17, 63966398. (26) Laforgue, A.; Bazuin, C. G.; Prud’homme, R. E. Macromolecules 2006, 39, 6473-6482. (27) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer, M. A. J. Am. Chem. Soc. 2002, 124, 12761-12773. (28) Rzayev, J.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 13373-13379. (29) Bailey, T. S.; Rzayev, J.; Hillmyer, M. A. Macromolecules 2006, 39, 8772-8781.

Li et al.

BCP-based nanopores will provide the fundamental basis for controlling the efficiency of nanowire fabrication and filtration and also will open possibilities to apply such materials for other purposes, such as catalysis and separation membranes. In this paper, cyclic voltammetry (CV) was used to address the above two issues on thin PS-b-PMMA films (PMMA volume fraction of 0.3) containing cylindrical PMMA domains.1,2 Quantitative analysis of the CV data provided a means for monitoring the completeness of the chemical etching of the cylindrical PMMA domains from a thin PS-b-PMMA film and also for exploring the surface functional groups on the resulting nanopore surface. CV data obtained from cylindrical nanopores with nanoscale radius (∼10 nm) showed the reduction of effective pore radius that reflects the thickness of the electrical double layer from the charged nanopore surface. Experimental Section Chemicals and Materials. All solutions were prepared with water having a resistivity of 18 MΩ cm or higher (Barnstead Nanopure Systems). PS-b-PMMA (Mn ) 39 800 g/mol for PS and 17 000 g/mol for PMMA and Mw/Mn ) 1.06) was purchased from Polymer Source and used as received. Potassium nitrate (Fisher Chemical), potassium ferricyanide (Acros Organics), and 1,1′-ferrocenedimethanol (Fc(CH2OH)2; Aldrich Chemical) were of reagent grade quality or better and used without further purification. Gold-coated silicon wafers, which were prepared by sputtering 10 nm of Ti followed by 200 nm of Au onto Si(100) wafers, were purchased from LGA Thin Films (Foster City, CA). Preparation of PS-b-PMMA Films. Gold substrates were cleaned in a Novascan PSD-UVT UV-ozone system for 15 min prior to use. A thin film of PS-b-PMMA was prepared on the gold substrate via spin-coating (2000 rpm) from its toluene solution (0.7% w/v) and was then annealed at 170 °C in vacuum (ca. 0.3 Torr) for 60 h to form cylindrical PMMA domains in the film. The PMMA domains were degraded via UV irradiation using a Novascan PSDUVT UV-ozone system (ca. 20 mW/cm2) under an Ar atmosphere, which involves simultaneous cross-linking of the PS matrix and degradation of the cylindrical PMMA domains.13,14 Subsequently, the degraded PMMA domains were removed by rinsing with glacial acetic acid (AcOH) for 2 min13,14 The thickness of a PS-b-PMMA film was measured using a J. A. Woollam alpha-SE spectroscopic ellipsometer. The ellipsometric thicknesses of annealed PS-b-PMMA films were ca. 20-30 nm prior to the UV irradiation. Atomic Force Microscopy (AFM) Measurements. AFM images were obtained by tapping-mode imaging in air, using a Digital Instruments Multimode AFM with Nanoscope IIIa electronics. Tapping mode AFM probes from Applied Nanostructures (cantilever length, 125 µm; force constant, 40 N/m; resonant frequency, 300 kHz) were used. Fourier Transform Infrared External Reflection Spectroscopy (FTIR-ERS). FTIR-ERS measurements of thin PS-b-PMMA films were performed in dry air using a Nicolet Protege 460 spectrophotometer equipped with a Harrick Seagull reflection accessory (Pleasantville, NY) and a TGS detector. All spectra were the sum of 256 scans obtained at 2 cm-1 resolution using p-polarized light at an 80° angle of incidence with respect to the gold substrate. Spectra of the PS-b-PMMA-coated gold substrates were obtained by subtracting the spectrum of a cleaned gold substrate. Electrochemical Measurements. CV measurements were performed in a three-electrode cell (Scheme 1) containing a Ag/AgCl (3 M KCl) reference electrode and a Pt counter electrode using a CH Instruments model 618B electrochemical analyzer. A polymercoated gold substrate (serving as the working electrode) was immobilized at the bottom of the cell. The diameter of the film area in contact with the solution, which was defined by a circular hole formed on the upper glass plate, was 0.65 cm. The pH of the solution was adjusted by adding a dilute solution of KOH or HCl and supporting electrolyte.

Nanoporous Films Fabricated from PS-b-PMMA

Langmuir, Vol. 23, No. 25, 2007 12773 Scheme 2

Figure 1. Tapping-mode AFM images of the surfaces of a thin PS-b-PMMA film (ca. 20 nm thick) on a gold substrate (a) after the annealing at 170 °C in vacuum (ca. 0.3 Torr) for 60 h and (b) after the removal of the PMMA domains via UV irradiation (10 min) and subsequent AcOH treatment.

Figure 2. FTIR external reflection spectra of PS-b-PMMA thin films (ca. 30 nm thick) on gold substrates (a) before UV irradiation and after UV radiation for (b) 5 min, (c) 10 min, and (d) 15 min. The samples were treated in AcOH for 2 min after being irradiated by UV light.

Results and Discussion Characterization of PS-b-PMMA Films before and after Removal of the PMMA Domains Using AFM and FTIRERS. Prior to electrochemical measurements, PS-b-PMMA films on gold substrates were characterized using AFM and FTIRERS methods. Figure 1 shows tapping-mode AFM images of a PS-b-PMMA film (ca. 20 nm thick) on a gold substrate (a) before and (b) after PMMA etching.13,14 In Figure 1a, circular bumps having radius of 10.6 ( 1.5 nm were observed, suggesting that the PMMA domains were oriented perpendicular to the film surface.3 After UV irradiation (10 min, 20 mW/cm2) and subsequent AcOH treatment, the circular bumps changed to circular depressions with 10.5 ( 1.4 nm radius due to the removal of the PMMA domains (Figure 1b). Very similar AFM images were obtained for PS-b-PMMA films in the thickness range of 20-30 nm. As shown in Figure 1, AFM could be used to observe the removal of PMMA domains from a PS-b-PMMA film surface. However, it is not possible to discuss whether the PMMA domains were removed completely from the film so that the resulting nanopores reached the bottom gold surface. The removal of the PMMA domains could also be observed using FTIR-ERS. Figure 2 shows FTIR-ERS spectra of PS-bPMMA films obtained before and after UV-AcOH treatment. The spectrum of a PS-b-PMMA film prior to UV-AcOH treatment (Figure 2a) exhibits absorption bands characteristics of PMMA and PS, including 1731 cm-1 (CdO of the ester groups in PMMA), 1188 and 1145 cm-1 (C-O stretching of the ester groups in PMMA), and 3023 cm-1 (-CH- stretching from benzene rings in PS). Upon increasing the UV irradiation time, the intensities of the bands assigned to PMMA decreased, whereas

those of PS did not change significantly (Figure 2b-d). UV irradiation for 15 min degraded most of the PMMA segments in the film, and thus the PMMA domains could be removed by subsequent AcOH treatment. However, the peak at 1731 cm-1 was found in FTIR-ERS spectra of the PS-b-PMMA films even after UV irradiation for 15 min (Figure 2d). This peak is ascribed to the ester carbonyl groups of residual PMMA units linked with the PS domains. Peaks due to other carbonyl groups (e.g., 1760 cm-1 for monomeric -COOH, and 1710 cm-1 for dimers)30 were not observed in these spectra. The peak intensity of the ester carbonyl band in Figure 2d suggests that the vast majority of the PMMA (g90%) was degraded and removed from the film by UV irradiation for 15 min and subsequent AcOH treatment. These results indicate that FTIR spectra can be used to assess the extent of the removal of the PMMA domains in a PS-bPMMA film. However, again, FTIR spectra cannot be used to assess the exposure of the gold surface at the bottom of the nanoporous film. Electrochemical Characterization of the Removal of the PMMA Domains in a PS-b-PMMA Film. Both AFM and FTIRERS provided conclusive evidence for the removal of the PMMA domains from PS-b-PMMA films. However, it was not possible to discuss whether the resulting cylindrical nanopores formed in the process reached the gold surface to form the RNE structure (Scheme 2b). In contrast, electrochemical methods provide a simple means for characterizing the RNE structure, as discussed below. In this study, the PS-b-PMMA films were deposited directly on cleaned gold substrates without a buffer layer used to balance the affinities of PS and PMMA. If the gold surface is preferentially wetted by the PS, the nanopores formed will not reach the bottom gold, because PS will form an insulating layer that cannot be removed via UV-AcOH treatment (Scheme 2a).3 The presence of the insulating layer will affect the redox current in CV, because the layer will inhibit the electrode reaction of redox species in solution. On the other hand, if the PMMA wets the gold surface, the resulting nanoporous film cannot be stably immobilized on the gold surface (Scheme 2c).3 In this case, the redox current will be similar to that on a bare gold electrode because of the detachment of the nanoporous film from the electrode surface. Figure 3a shows CVs of 3.0 mM Fc(CH2OH)2 in 0.1 M KNO3 on gold substrates coated with PS-b-PMMA films (ca. 30 nm thick) after UV irradiation for the specified time (tUV). The redox currents of Fc(CH2OH)2 increased for longer irradiation times. UV irradiation for 2 min did not give a peak-shaped CV because (30) Nakanishi, K.; Solomon, H. Infrared Absorption Spectroscopy; HoldenDay: San Francisco, CA, 1977.

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the PMMA domains were likely removed completely (Figure 3). We are currently investigating the origins of this apparent inconsistency between our results and the theory. However, there are a number of papers that have reported peak-shaped CVs with smaller peak currents for nanoporous anodic alumina membranes32 and self-assembled monolayers38,39 in the presence of overlapped diffusion layers. Interestingly, the peak currents in the plateau region shown in Figure 3 were similar to that (49 µA) estimated using eq 131 from the total area of the gold disk electrodes exposed by the nanopores

ip ) 0.446nFAC Figure 3. (a) CVs (scan rate 0.05 V/s) of 3.0 mM Fc(CH2OH)2 in 0.1 M KNO3 on gold electrodes coated with thin PS-b-PMMA film (ca. 30 nm thick) at different UV irradiation times. The samples were treated in AcOH for 2 min after being irradiated by UV light. (b) Relationship between UV irradiation time and anodic peak current in CVs of 3 mM Fc(CH2OH)2. The plots and error bars indicate the averages and standard deviations obtained from at least three separate electrodes. The average peak current obtained on bare gold electrodes (gray solid line) and theoretical peak current calculated from the average pore diameter (21 nm) and pore density (900 pores/µm2) obtained from Figure 1 (dashed line) are also shown.

xnFDV RT

(1)

of the polymer layer remaining on the gold surface, which inhibited the electrode reaction of Fc(CH2OH)2. CVs for tUV g 10 min are quasireversible (the potential difference between oxidation and reduction peaks (∆Ep) is ∼100 mV) with similar peak current (ip) for the oxidation of Fc(CH2OH)2, suggesting that the PMMA domains were almost completely removed from the PS-b-PMMA films to form a RNE structure. As shown in Figure 3b, the ip value reached a plateau for films irradiated for 10-15 min. The ip values at the plateau were smaller than that on bare gold electrodes, which is indicated by the gray solid line in Figure 3b). If the PMMA domains are completely removed to form the RNE structure (Scheme 2b), ip will be determined by the crosssectional area defined by the nanopores. The AFM image shown in Figure 1b gives the pore radius (a ) 10.5 nm) and density (∼900 pores/µm2). The pore density corresponds to 36 nm as the average spacing between adjacent nanopores, assuming that the nanopores are packed in a hexagonal arrangement. The pore spacing is much smaller than the thickness of the diffusion layer on the time scale of the CV measurements (e.g., ∼20 µm thick for CV at 0.1 V/s scan rate).31-33 Diffusion layers developed from individual nanopores will thus overlap on the time scale of the CV measurements, resulting in peak-shaped CVs.33-35 In addition, since the thickness of the film, i.e., pore length, was very small (∼30 nm) as compared with the thickness of the diffusion layer, the CVs obtained were very similar to those obtained with nanodisk electrode arrays with no recession.31,36 According to theory,36 the peak current expected from overlapped diffusion layers should be similar to that calculated from the geometrical area with no insulating layer, which was supported by previous reports.37 In contrast, we reproducibly observed peak-shaped CVs with smaller peak currents even after

where n is the number of electrons (n ) 1 for the redox-active molecules used in this study), F is Faraday’s constant (96485 C/mol), R is the gas constant (8.31 J/K mol), T is temperature (298 K), A is the electrode area [Nπa2; a is the average radius of nanopores; N is the number of exposed nanopores, and can be calculated from the density of the pores (900 pores/µm2) and electrode area defined by the circular hole in the glass cell (0.65 cm in diameter, Scheme 1)], D is the diffusion coefficient of the redox-active species [7.6 × 10-6 cm2/s for Fe(CN)63- and 6.4 × 10-6 cm2/s for Fc(CH2OH)2],31,41 and C (mol/cm3) is its concentration. As expected from eq 1, a linear relationship was obtained on our nanoporous electrodes between ip and V1/2 for both redox molecules at V e 0.1 V/s. Furthermore, the slope of the line obtained is similar to that estimated from the active electrode area, diffusion coefficient, and concentration of the redox species in each case. This agreement suggests that, at tUV g 10 min, the PMMA domains of the PS-b-PMMA film were almost completely degraded and removed to expose the bottom gold surface, which is consistent with the FTIR results discussed above (Figure 2). The polymer layer remaining on the gold electrode at the bottom of the nanopores after the PMMA degradation (see Scheme 2a) would be negligible, as suggested by the very small influence of pH on the reversibility of the CVs (vide infra). In addition, the reproducible and repeatable observation of the CVs shown in Figure 3 indicates that the nanoporous film strongly adhered to the gold surface without the detachment of the nanoporous film from the gold substrate (see Scheme 2c). The vertical orientation of the PMMA domains in the original PS-b-PMMA film on the bare gold was probably obtained due to roughness-induced orientation,42 considering the gold surface contains grains having average diameters of ca. 30 nm and a root-mean squares roughness of ca. 2 nm (data not shown). These results indicate that CV can be used to assess the exposure of the bottom electrode as a result of the chemical etching processes of the PMMA domains and also to confirm the vertical orientation of the etchable domains. Exploration of Surface Functional Groups on Nanopores Prepared from PS-b-PMMA Using pH-Dependent CV Measurements. Mass transport of charged species through charged nanoscale pores is significantly influenced by electrostatic interactions.43-47 Here, CV was used to characterize the surface

(31) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; pp 619-623. (32) Brumlik, C. J.; Martin, C. R.; Tokuda, K. Anal. Chem. 1992, 64, 12011203. (33) Ito, T.; Audi, A. A.; Dible, G. P. Anal. Chem. 2006, 78, 7048-7053. (34) Arrigan, D. W. M. Analyst 2004, 129, 1157-1165. (35) Zoski, C. G.; Yang, N.; He, P.; Berdondini, L.; Koudelka-Hep, M. Anal. Chem. 2007, 79, 1474-1484. (36) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51. (37) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762766.

(38) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (39) Grancharov, G.; Khosravi, E.; Wood, D.; Turton, A.; Kataky, R. Analyst 2005, 130, 1351-1357. (40) Brookes, B. A.; Davies, T. J.; Fisher, A. C.; Evans, R. G.; Wilkins, S. J.; Yunus, K.; Wadhawan, J. D.; Compton, R. G. J. Phys. Chem. B 2003, 107, 1616-1627. (41) Fan, F.-R. F. J. Phys. Chem. B 1998, 102, 9777-9782. (42) Sivaniah, E.; Hayashi, Y.; Matsubara, S.; Kiyono, S.; Hashimoto, T.; Fukunaga, K.; Kramer, E. J.; Mates, T. Macromolecules 2005, 38, 1837-1849. (43) Wei, C.; Bard, A. J.; Feldberg, S. W. Anal. Chem. 1997, 69, 4627-4633. (44) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M.; Lee, S. B. AdV. Mater. 2001, 13, 1351-1362.

Nanoporous Films Fabricated from PS-b-PMMA

Figure 4. CVs (scan rate 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6 and (b) 3.0 mM Fc(CH2OH)2 in 0.01 M KNO3 on a gold electrode coated with an etched PS-b-PMMA film (29 nm in film thickness) at pH 3.7, 4.6, 5.5, 6.3, 7.6, and 10.5. (c) Relationship between pH and anodic and cathodic peak currents of Fc(CH2OH)2 (open circles) and Fe(CN)63- (filled circles), respectively, obtained using the electrode. The sample was irradiated by UV light for 15 min and subsequently treated by AcOH for 2 min.

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Figure 5. CVs (scan rate 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6 and (b) 3.0 mM Fc(CH2OH)2 in 0.1 M KNO3 on a PS-b-PMMA-coated gold electrode at pH 3.7, 4.6, 5.5, 6.3, 7.6, and 10.5. (c) Relationship between pH and peak current of Fc(CH2OH)2 (open circles) and Fe(CN)63- (filled circles) obtained from the electrode. The data shown in Figure 5 were obtained using the same electrode that gave the data shown in Figure 4.

charge on cylindrical nanopores in an etched PS-b-PMMA film. Figure 4a shows CVs of Fe(CN)63- on a PS-b-PMMA-based RNE in 0.01 M KNO3 at different pH conditions. The redox current of Fe(CN)63- was smaller at higher pH, whereas the change in redox current with pH was very small (e3%) under the same conditions on bare gold electrodes (data not shown). In contrast, CVs of Fc(CH2OH)2 on the same RNE were very similar at the three pH conditions employed (Figure 4b). As summarized in Figure 4c, the ip from Fe(CN)63- changed in the pH range from 4.6 to 6.3 and reached a plateau from pH 6.3 to 10.5. The ip was similar at pH 3.7 and 4.6. The difference in the pH-dependence of CVs obtained from the anionic and uncharged redox species suggests the presence of ionizable functional groups on the nanopore surfaces:45,46 At pH e4.6, the functional groups on the nanopore surface were uncharged, and thus Fe(CN)63could pass through the nanopores freely (Scheme 3a). However, at pH g6.3, the nanopore surface was negatively charged due to the deprotonation of the functional groups. Owing to electrostatic repulsion by the nanopore surface, Fe(CN)63- can only pass through a limited portion of the nanopore defined by the electrical double layer from the nanopore surface (Scheme

3b). The similar ∆Ep obtained under the different pH conditions for Fe(CN)63- (Figure 4a) indicates that the gold surface at the bottom of the nanopores was not covered with charged polymer residues. Thus, the electrostatic effects originated from the charged groups on the nanopore surface rather than the electrode surface.45,46 In contrast, ∆Ep in CVs of charged redox species on electrodes modified with self-assembled monolayers having ionizable terminal groups shifts due to a change in the electrode reaction kinetics originating from electrostatic effects between the redox species and electrode surface.48,49 The pH range that showed changes in the redox current for Fe(CN)63- suggests the presence of surface -COOH groups48,50 within the nanopores. The surface -COOH groups are probably formed during UV irradiation in the presence of trace oxygen. The FTIR-ERS spectra did not give any bands corresponding to the surface -COOH groups (Figure 2d) on the nanopore surface due to the insufficient sensitivity of this method. Amidation could be used to tailor the surface of etched PS-b-PMMA nanopores,51 supporting the presence of the surface -COOH groups. Effect of the Electrical Double Layer on CVs of PS-bPMMA-Based RNE. The mechanism shown in Scheme 3 is supported by CV measurements at different supporting electrolyte concentrations. Figures 5 and 6 show CVs of (a) Fe(CN)63- and (b) Fc(CH2OH)2 in 0.1 and 1 M KNO3 solutions, respectively, at different pH values. Figures 5c and 6c summarize the relationship between pH and ip at these electrolyte concentrations. As with CVs in 0.01 M KNO3 (Figure 4), the ip for Fe(CN)63decreased from 4.6 to 6.3 and reached a plateau at higher pH, whereas that for Fc(CH2OH)2 did not change, regardless of pH. The shapes of the CVs for Fe(CN)63- were very similar at the different pH conditions, for the same supporting electrolyte concentration. These pH-dependent behaviors are consistent with

(45) Wang, G.; Zhang, B.; Wayment, J. R.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 2006, 128, 7679-7686. (46) Wang, G.; Bohaty, A. K.; Zharov, I.; White, H. S. J. Am. Chem. Soc. 2006, 128, 13553-13558. (47) Kuo, T.-C.; Sloan, L. A.; Sweedler, J. V.; Bohn, P. W. Langmuir 2001, 17, 6298-6303.

(48) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767-2775. (49) Schon, P.; Degafa, T. H.; Asaftei, S.; Meyer, W.; Walder, L. J. Am. Chem. Soc. 2005, 127, 11486-11496. (50) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006-2015. (51) Li, Y.; Ito, T. Unpublished results.

Scheme 3

12776 Langmuir, Vol. 23, No. 25, 2007

Li et al.

KNO3 at pH 7.6 and 3.7. At pH 7.6 and 3.7, the nanopore surface would be negatively charged and uncharged, respectively, because the pH in these two cases was higher and lower than the transition pH range that shows the change in the redox current of Fe(CN)63-. The reff values obtained from CVs of Fe(CN)63- at pH 7.6 are clearly smaller than those obtained at pH 3.7. The difference in reff is also larger at the lower supporting electrolyte concentration. In contrast, reff obtained from CVs of Fc(CH2OH)2 is very similar at pH 7.6 and 3.7. The pH-dependent changes in reff for CVs of Fe(CN)63- were reasonably close to distances reflecting the extension of the electrical double layer. The distances were in the range of 2dD and 3dD,45 where dD is the Debye length of each solution (Table 1) calculated using eq 254

dD )

(∑ i

Figure 6. CVs (scan rate 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6 and (b) 3.0 mM Fc(CH2OH)2 in 1 M KNO3 on a PS-b-PMMA-coated gold electrode at pH 3.7, 4.6, 5.5, 6.3, 7.6, and 10.5. (c) Relationship between pH and peak current of Fc(CH2OH)2 (open circles) and Fe(CN)63- (filled circles) obtained from the electrode. The data shown in Figure 6 were obtained using the same electrode that gave the data shown in Figures 4 and 5. Table 1. Effective Pore Radius (nm) of PS-b-PMMA Nanopores Measured from CVs at pH 7.6 and 3.7 at Different Supporting Electrolyte Concentrationsa 1M KNO3

0.1 M KNO3

0.01 M KNO3

Fe(CN)63-, pH 3.7b Fe(CN)63-, pH 7.6b Fc(CH2OH)2, pH 3.7b Fc(CH2OH)2, pH 7.6b

13.4 ( 2.1 12.3 ( 2.0 12.0 ( 0.2 11.7 ( 0.2

12.2 ( 1.8 10.4 ( 1.3 10.7 ( 0.2 10.5 ( 0.2

9.6 ( 0.6 6.9 ( 0.2 9.6 ( 0.2 9.6 ( 0.1

dD (nm)c

0.3

0.9

1.8

a

The samples were irradiated by UV light for 15 min and subsequently treated in AcOH for 2 min. The scan rate was 0.02 V/s. KNO3 was used as the supporting electrolyte. The concentration of the redox-active species is 3.0 mM. b The data calculated using eq 1 are the averages and standard deviations measured from at least three separate samples. c Debye length calculated from the concentrations of the supporting electrolyte and redox species using eq 2.

the mechanism illustrated in Scheme 3. On the other hand, the ip value was smaller at lower supporting electrolyte concentration for both Fe(CN)63- and Fc(CH2OH)2. This decrease in ip may be partly ascribed to increased uncompensated resistance due to the lower solution conductivity at the lower supporting electrolyte concentration,52 but this contribution is believed to be minor, as suggested by the small difference in ip (